The invention relates to a high-pressure discharge lamp provided with a discharge vessel enclosing a discharge space and having a ceramic wall, which vessel is provided near at least one end with a ceramic end plug in which a current-supply member of a main electrode is arranged in a gas-tight manner by means of a sealing ceramic connection. The end plug is connected by means of a sintered joint to the wall of the discharge vessel in a sunken position with respect to the end of the vessel, which sintered joint between the end plug and the wall of the vessel is covered by sealing ceramic on the side facing the end of the vessel. The invention further relates to a method of obtaining such a lamp.
The term ceramic wall, ceramic end plug or ceramic filling piece as used hereinafter is to be understood to mean herein both a material consisting of polycrystalline metal oxide, such as, for example, densely sintered aluminium oxide, and monocrystalline metal oxide, such as, for example, sapphire. When used as the wall of the discharge vessel, the ceramic material is translucent. The term sealing ceramic used in this description and these claims is intended to mean a connection material having a lower softening temperature than the material of the wall of the discharge vessel, which may be present in the glass phase, or in the cyrstalline phase, or in a combination of these two phases. The discharge vessel contains an ionizable filling, which in general comprises sodium, mercury and a rare gas or a metal halide, mercury and a rare gas.
A lamp of the kind mentioned in the opening paragraph is known from Netherlands Patent Application No. 7704135 and corresponding U.S. Pat. No. 4,198,586. The known lamp is widely used and its discharge vessel has a satisfactory reproducible gas-tight construction. However, in order to obtain such a gas-tight seal, the space outside the discharge space defined by the end of the vessel, the end plug and the current-supply member is partly filled in such a manner that the sintered joint between the wall of the discharge vessel and the end plug is covered by sealing ceramic. In practice, the use of a comparatively large quantity of sealing ceramic leads to the sealing ceramic in the space between the end plug and the current-supply member extending also into the discharge space enclosed by the discharge vessel. Sealing ceramic in the discharge space has proved to be disadvantageous because under the influence of the temperature prevailing in the operating conditions of the lamp it reacts with filling constituents of the discharge vessel. The occurrence of such reactions generally leads to shortening of the life of the lamp.